1. Field of the Invention:
The present invention relates to improved synthetic amorphous silicas particularly adapted for the reinforcement of elastomeric materials, especially tires. The present invention also relates to methods for the preparation of these new synthetic amorphous silica materials.
2. Description of the Prior Art:
Reinforcement materials for elastomers, particularly tires, have hitherto been based upon the carbon blacks. Recent attention has been given the feasibility of replacement of carbon black reinforcement by various silica materials, since the latter have been found to provide sufficient tearing resistance and satisfactory traction or frictional stability on wet or icy surfaces when incorporated in a tire tread. While generally noted as an adequate replacement for carbon blacks, unfortunately the incorporation of silica materials has been accompanied by a concomitant trading of important properties observed in tires employing the same in the treads. For example, silica materially increases the tendency for tire treads to heat during use and also tends to reduce the abrasion resistance thereof. Certain disadvantages have been minimized by the additions of various coupling agents, such as silanes, but again not without the need to suffer other significant disadvantages such as, for example, an unpleasant odor, increased production costs, and scorching of the elastomer at temperatures encountered during use of the tire.
Ideally, silicas to be employed in replacement of carbon black reinforcement additives for tires should provide all of the advantages of the latter without suffering these significant deficiencies. Initial efforts in this regard have centered on the establishment of a simple relationship between specific areas (BET) of the silica and its behavior in vulcanized materials.
Accordingly, the U.S. Pat. No. 3,235,331 discloses a method for preparing silicas for the express purpose of reinforcing elastomers wherein the silica material is prepared with an eye toward the control of its specific area. Thus, the patentees disclose a method of producing finely divided siliceous pigments by reacting aqueous alkali metal silicate solutions, such as aqueous sodium silicate solutions, with an acidification agent to yield a precipitate having a surface area on the order of 200 m.sup.2 /g.
Other efforts, such as those described in U.S. Pat. No. 3,445,189, have correlated specific area and oil-absorption characteristics of silica particulates adapted for the reinforcement of, e.g., rubber. In this regard, the patentees indicate that the finely divided silica must possess a surface area of from 100 to 250 m.sup.2 /g and an oil absorption of at least 2 cm.sup.3 /g.
Further along these lines, U.S. Pat. No. 2,731,326 provides correlation between specific surface area and oil-absorption capacities of dense amorphous silica aggregates. The patentees disclose a process to yield a silica in the form of aggregates having a particle size greater than that evident in the colloidal state and a specific area of from 60 to 400 m.sup.2 /g. The particles exist as spheriods of dense amorphous silica of substantially uniform size, linked in an open reticulated structure which is uniformly reinforced with dense amorphous silica such that, after drying, the oil-absorption index (expressed in milliliters per 100 grams of solid substance) lies between 1 and 3 times greater than the value of the specific area expressed in m.sup.2 /g (BET). In providing particles characterized in this fashion, the patentees further indicate the necessity of specifically limiting:
(a) the size of the ultimate spheroidal units; PA1 (b) the openness of packing of the ultimate units in the reticulated aggregate structure; PA1 (c) the extent to which the structure is reinforced; and, PA1 (d) the uniformity of the structure.
Consequently, careful process control is mandated.
In French Pat. No. 1,506,330, some of the present invention disclose a method for producing particulate silicas adapted for use as additives, particularly in rubbers, which particulates are characterized as possessing a relatively high surface area (BET) on the order of about 300 m.sup.2 /g, and further characterized as having a roughness factor of between about 2.5 and about 3.5, this latter characteristic being determined as the ratio of the specific area (BET) to the area calculated by electron microscopy techniques. Accordingly, yet another factor has been determined significant with respect to the parameters necessary for insuring the commercial utility of silica particulates as reinforcement for elastomeric materials.
Still another criterion deemed important is the water content of the filtration cake formed during preparation of the silica particulate, and its correlation with the behavior of these materials in rubbers. Thus, there must be differentiated silicas having a high water content and those having a low water content, designated as high-structure silicas and low-structure silicas, respectively.
The art recognizes that still further physical and chemical characteristics are of extreme importance regarding the use of silicas in the reinforcement of elastomers. However, there is no generally accepted agreement as to the relative importance of the various phenomena responsible for the reinforcement of elastomers. See, for example, Dannenberg et al, R.G.C.P., v. 51, n. 5, 1974.
significantly, also, is the inability to predict, from first principles, the manner in which various silicas might replace carbon blacks in the reinforcement of elastomers. Differences in behavior may be attributed to, inter alia, the morphological differences between these two differing types of additives, which precludes the ability to extrapolate data from one to another. Accordingly, predictions based upon specific areas and oil-absorption indices, generally employed as indicators of the behavior of carbon blacks in elastomers, typically lead to contradictory results respecting silica replacements therefor. For example, the oil index (DBP number) of carbon blacks ranges from about 40 to about 140 cm.sup.3 /100 g, while the oil index of silicas may be as high as about 500 cm.sup.3 /100 g. While this information would reasonably lead to a postulate that these silicas possess considerable reinforcing action, actual observations fall to confirm the same. Further anomalous results regarding silicas is the fact that silica particulates having considerably different specific areas (BET) and oil-absorption capacities may well behave in similar manners in a given rubber material.
Therefore, the need exists to provide a silica particulate specifically adapted as a reinforcement additive for elastomeric materials and a commercially-acceptable method therefor. The need also exists to provide such materials and processes which may consistently be employed with reproducible and predictable results.